1. Field of the Invention
The present invention relates to a level transducer and more particularly to a level transducer having a self-modulation function to increase accuracy of the level transducer.
2. Description of Related Art
Level transducers are commonly used in many industries such as food industries, chemical industries, and petrochemical industries to monitor a level change of material stored in a tank to confirm amount or inventory status of the material.
With reference to FIGS. 5 and 6, a conventional level transducer comprises a tuning fork unit 60, a control unit 70 and a power supply unit 71. The tuning fork unit 60 has a tuning fork 61, a piezoelectric driving element 62 and a piezoelectric sensing element 63, wherein the piezoelectric driving element 62 and the piezoelectric sensing element 63 are sleeved on the tuning fork 60, and the piezoelectric driving element 62 is stacked on the piezoelectric sensing element 63. The control unit 70 is electrically connected to the power supply unit 71, wherein the control unit 70 and the power supply unit 71 are respectively connected to the piezoelectric sensing element 63 and the piezoelectric driving element 62.
When the level transducer is in use, the power supply unit 71 produces a square wave driving voltage signal VT to the piezoelectric driving element 62. The piezoelectric driving element 62 obtains the driving voltage signal VT and deforms. The tuning fork 61 vibrates with a deformation of the piezoelectric driving element 62. Since the tuning fork 61 is inserted in a stored material, a vibration frequency of the tuning fork 61 is influenced by the nature and a level of the stored material, and the vibration frequency of the tuning fork 61 further influences a frequency of the deformation of the piezoelectric driving element 62.
With further reference to the FIG. 7A, when in a positive voltage period of a function cycle of the power supply unit 71, the piezoelectric driving element 62 obtains a positive voltage and expands. The piezoelectric sensing element 63 is extruded due to the expansion of the piezoelectric driving element 62 and shrinks.
With further reference to the FIG. 7B, when in a negative voltage period of the function of the power supply unit 71, the piezoelectric driving element 62 obtains a negative voltage and shrinks. The piezoelectric sensing element 63 expands with the shrinking piezoelectric driving element 62.
A sine wave sensing voltage signal VR is produced by the piezoelectric sensing element 63 by a deformation of the piezoelectric sensing element 63 and is sent to the control unit 70. A frequency of the sensing voltage signal VR is based on a frequency of the deformation of the piezoelectric sensing element 63. When the tuning fork 61 is exposed in air, the frequency of the sensing voltage signal VR approximately equals a frequency of the driving voltage signal VT. When the tuning fork 61 is inserted in a stored material, though the frequency of the driving voltage signal VT is not changed, the vibration of the tuning fork 61 slows down due to a resistance of the stored material, and the frequency of the deformation of the piezoelectric driving element 62 is also decreased. Therefore, the frequency of the deformation of the piezoelectric sensing element 63 and the frequency of the sensing voltage signal VR are both decreased. The decreases of the sensing voltage signal VR are based on the nature and a level of a stored material in which the tuning fork 61 inserted, such that the control unit 70 can obtain the nature and the level of the stored material by calculating the sensing voltage signal VR.
By the above described, the deformation of the piezoelectric driving element 62 is controlled by the driving voltage signal VT, and the frequency of the deformation of the piezoelectric driving element 62 can be changed by adjusting the driving voltage signal VT. However, when the function cycle of the voltage signal VT transits from the positive voltage period to the negative voltage period, the piezoelectric driving element 62 keeps expanding at the transition moment, and thus, the piezoelectric element 62 does not shrink immediately at the transition moment due to inertia of the expanding. Thus, the vibration of the tuning fork 61 becomes weaker, and oscillation amplitude of the sensing voltage signal VR becomes smaller. The control unit 70 obtains sensing results of the stored material based on the inaccurate sensing voltage signal VR, and thus, accuracy of the sensing results obtained by the control unit 70 is reduced.